zImage

zImage is a compressed Linux kernel image format used by many bootloaders to load the kernel into memory. It consists of a small bootstrap routine and a compressed kernel payload. When a system boots, the bootloader copies the zImage into memory and transfers control to it; the kernel then decompresses itself in place and begins execution. The image is typically stored on disk as a single file and is consumed by the bootloader during the early boot phase.

The term zImage is historically tied to the Linux kernel’s image naming. The “z” denotes a compressed, or “small,” image, in contrast with the larger bzImage (“big zImage”) used for systems requiring more memory. On many architectures, the choice between zImage and bzImage corresponds to the amount of memory the kernel expects to occupy after decompression; zImage targets smaller configurations, while bzImage supports larger kernels and higher memory footprints. Both formats are produced by the kernel build process and are Arch/CPU dependent.

Generation and formats: The kernel build system can produce a zImage or a bzImage, depending on configuration and architecture. The image is generally gzip-compressed, though modern kernels can be configured to use other compression methods for the final image. In distribution packaging, the kernel file may be named vmlinuz, which is a gzipped kernel image and may internally be a zImage or bzImage, depending on how the build was configured.

Usage and deployment: In traditional PC boot setups, bootloaders such as GRUB load the kernel image directly from /boot. In embedded systems, a common workflow is to wrap the image in a U-Boot uImage container for boot loading. After loading, the image decompresses and the Linux kernel takes over hardware initialization and system startup.

In summary, zImage is a compact, compressed kernel image designed for straightforward bootloading, with bzImage serving as its larger counterpart for systems with more memory.